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Fun with Transistors 


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Fun with Transistors 

Written By: Steven Robert Cypherd 


Playing with transistors lets you in on what makes just about everything in our world work. 
Transistors are easy to work with, requiring just a few basic steps. In most cases you need 
to use biasing resistors to control when they turn on and off. Biasing resistors also lower the 
current needed to switch a transistor on and off. Biasing keeps a transistor in one of its 
states so it only changes states when you want it to. Biasing helps with signal flow. Generic 
transistors need biasing. Transistors are current devices and voltage only plays a small part 
in their operation. 

Resistor 1 Main biasing resistor keeps the transistor in the Off or On state. Changing this 
resistor's value adjust the turn On voltage and current. 

Resistor 2 Input voltage and current adjustment to lower the load on what is driving the 
transistor. Transistors can change state with as little as 2 mA of current. 

Resistor 3 Protects the load the transistor is driving. LEDs need a current-limiting resistor to 
protect them. 

You can get packs of transistors at Radio Shack. 

NPN (Negative-Positive-Negative): Switches negative voltages with positive voltage control. 
It has the effect of an inverter. 

PNP (Positive-Negative-Positive): Switches positive voltages with negative voltage control. 
Also an inverter. 

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Fun with Transistors 

MOSFET: Basic performance is like an NPN transistor, but they are much more. 

Parts of a transistor MOSFET 

Emitter Input Voltage Source 

Collector Output Voltage Drain 

Base Controls the states of the transistor. -- Gate 

Transistors come in many formats so check the data sheets. Even generics like the 2N2222 
or the 2N7906 can handle about 30 mA or maybe up to ten LEDs. They can drive small 
motors on small vehicles and small 5-volt relays. You can build oscillators, timed switches, 
animated things and much more. 

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Fun with Transistors 

Step 1 — Fun with Transistors 

Fun with Transistors 





All transistor work about the same. P is positive, N is negative, B is base, 
C is collector, E is Emitter. AAosfet E> is drain, & is gate, S is source. 
When you app!y a voltage to the base/gate the transistor allows current 
to flow through the emitter/source to the eolteetcr/drain.Vcu use Biasing 
resistors to control when the transitor changes states. 

Rl holds the transistor in the Off state. 

R2 protects the input source. 

R3 protects the Led. 

Transistors behave with correct Biasing. 

51 open output(c) is high Led is Off 

51 closed output(e) is low Led fs On 

Input High the Led is On 
Any microprocessor wifl drive this circuit. 

PNP s — N 

2N29Q7 Jj 

r Input Low 
*Led is On 

Mosfel ESI 70 same as an N FN. bul so much more 
ft Simple ^^^^k *i nw.ll 

Touch Switch £*^H *"* M f e 4i " k 

T'W ^^^^^^ Si open. oirtputfcji lo* LeJ 

Dual-H Bridge Motor Driver 

PNP M + 9 V+ M 

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2 Sl 3 OFF 




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HPM 2N2222A - 2H£ 2H2907A 
Diodes 1N-5317 Schottky 
Resistors lk 
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Da not short sides 

Clockwi s e 


L low-G | B v+ | z don't care 

Ik is 1000 ohms 

• Most microprocessors cannot drive more than 20 mA on one output pin at a time. This is 
where transistors come in to drive the outside world. Through biasing you can connect 
transistors to tri-state outputs. Hi-Z is one state of a tri-state output where the pin is 
floating. Biasing keeps a transistor from being affected by this type of open circuit. You 
must use a resistor between the microprocessor pin and the transistor to protect the 
microprocessor. Use 220 to 1000 ohms. 

• Look at the pictures and see that transistors are very easy to use. For each circuit you 
adjust the biasing resistors so that you drive the transistor with very little current and the 
load with maximum current. These very basic circuits will drive a lot of useful things. Our 
world is driven by transistors. Transistors do so much that there is a ton of information out 
there for you to do anything that you want to with them. 

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Fun with Transistors 

Step 2 

HALL Effect 



Orte of many Types 
[ 1 

A device. Some 
are transistors 

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Photo Transistors Darlington 

_ More Current *&— 




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Ed flL jxmrpflWBC supply ajid or jruur ^™ 

transistor. Eaen LED needs abam 2 i'Mls 

to ",i"n I. . Ahull 2 6 ml ji -i :..i-:l 

See Testing an LED 

12--I16 — ^o 

- I 

MosFet Gate & V+ is ON 
Secret Gate & Gnd is OFF 

Lock Anything is Possible 

MosFet BS170 i 
'Touch Switch 

- ■ [ m * * *** "* 3 Gnd ' • • ■ • 
«..--. *TT* . 2 Gate' 

- 1 V+ wmm^%\ 

www* mwwwM 

• Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) are fun. Creating a touch 
switch with them is easy. They remind me of vacuum tubes because the Gate is not 
directly connected to anything. 

• You can test bipolar transistors with a volt-ohm meter; between the Emitter and Base you 
should read a diode and between Collector and Base another diode. The MOSFET uses the 
principle of field effect, like a Hall Effect device/transistor that switches when you pass a 
magnet over it. MOSFETs seem rugged, but they are susceptible to static discharge so 
keep them in their grounded holder until they are ready for use. 

• The touch switch is simple, but powerful. MOSFETs can drive up to 500 mA and can 
handle about 60 volts. Do not use a touch switch when controlling more than 12 volts. Use 
another transistor or relay to drive a high-voltage circuit from the touch switch. The touch 
switch wires can be greatly extended. You can make a secret lock by placing metal tacks, 
thumbtacks, nails or wires on anything. If the surface is conductive, like metal, you will 
need to insulate your contacts. 

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Fun with Transistors 

Step 3 

My Battory Box 

4 AA holder 

Panel bus 

Hot Glue 

A. fcacrti Imm lot 1 



s> o o 

« 4.S 3 i.s a 

• Lighting multiple LEDs is easy; just add up the voltage and the current of each LED to 
match your power supply and/or your transistor. You connect them Cathode (-) to Anode 
(+) down the line. LEDs need about 2 volts and about 2-6 mA each to work. 12 volts lights 
about 6 LEDs. 

• My circuit is a mini LED lighting a mini LDR driving the base of the PNP transistor to light 
6 LEDs on 12 volts. The LDR is a Light Dependent Resistor. LDRs have a resistance of 
about 160kQ in the dark to about 25kQ in the light. 

• The biasing resistor worked out to be 2.2k connected to V+ and the Base of the transistor. 
You have to play with this to find your balance point to keep the transistor off until the 
LEDs lights. Generic transistors like the 2907 and 2222 have a wide operating window. I 
used a 10k-25k potentiometer to find my balance point. It is fun. You can light other strings 
of LEDs with this method. 

Transistors are fun. 

This document was last generated on 2012-11-01 03:28:02 PM. 

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